We present a method to describe the temperature dependence of emission Mˆssbauer spectra showing slow spin-lattice relaxations of Fe3+ in MgO single crystals, obtained after implantation of 57Mn at ISOLDE/CERN. The analysis is based on the Blume-Tjon model for the line-shape of relaxing paramagnetic sextets with the spin relaxation rate, τ-1 as a parameter. The temperature dependent spin relaxation rate of Fe3+ in MgO is found to increase to~ 108 s-1 at 647 K by assuming a relaxation rate of τ-1